X-Ray images using films have been conventionally widely used in clinical practice. However, X-ray images using films provide analog image information, and therefore are not sufficient in resolution and are also not favorable in storage properties. In recent years, digital systems such as computed radiography (CR) and flat panel type radiation detectors (flat panel detectors: FPDs) have been developed.
In indirect FPDs for converting radiation into visible light, radiological image conversion screens are used for converting radiation into visible light. Such a radiological image conversion screen includes a phosphor for X-ray, such as thallium-activated cesium iodide (CsI:Tl) or terbium-activated gadolinium oxysulfide (Gd2O2S:Tb) (GOS), and the phosphor for X-ray emits visible light according to irradiated X-ray and the light emitted is converted into an electric signal by a photodetector provided with TFT, CCD, and the like, thereby resulting in conversion of X-ray information into digital image information.
FPDs are demanded to be higher in sensitivity to X-ray radiated and higher in sharpness. While it is effective to increase the amount of a phosphor in a radiological image conversion screen in order to improve the sensitivity, the amount of the phosphor is increased, thereby causing an increase in the thickness of the radiological image conversion screen and causing visible light emitted from the phosphor to be more easily diffused, resulting in a reduction in sharpness. In order to suppress the influence of such light diffusion and enhance sensitivity with sharpness being kept, for example, Patent Literature 1 discloses a method where the average particle size, the filling rate, the thickness, and the like of a phosphor is adjusted.